The present invention relates to an exposure apparatus and an exposure method. In particular, the present invention relates to an exposure apparatus, especially a scanning exposure apparatus and a scanning exposure method in which a mask held by a mask stage and a photosensitive substrate held by a substrate stage are subjected to scanning in a synchronized manner by using an illumination light beam, so as to transfer, onto the photosensitive substrate, a pattern such as a semiconductor circuit pattern and a liquid crystal circuit pattern formed on the mask via a projection optical system.
Recently, a scanning type exposure apparatus based on the step-and-scan system (hereinafter referred to as xe2x80x9cSandS systemxe2x80x9d, if necessary), which realizes a resolution line width of not more than 0.5 xcexcm, has been developed as an exposure apparatus for producing semiconductor elements. The apparatus is being actively improved in order to realize genuine and practical use for the semiconductor production line. The exposure apparatus based on the SandS system is disclosed, for example, in (1) Japanese Laid-Open Patent Publication No. 56-111218, (2) SPIE Vol. 1088 Optical/Laser Microlithography II (1989), pp. 424-433, (3) Japanese Laid-Open Patent Publication No. 2-229423, and (4) Japanese Laid-Open Patent Publication No. 4-277612.
Among them, in order to use mirror projection of 1xc3x97 magnification in the SandS system, the above-mentioned patent document (1) discloses a system in which the mask is linearly moved in the scanning direction during scanning exposure, while the semiconductor wafer is moved in the scanning direction in a scanning manner, and it is moved in a direction perpendicular thereto in a stepwise manner. The above-mentioned document (2) discloses a reduction projection scanning exposure apparatus of the SandS system which uses a 1/4 reduction projection optical system having a circular arc-shaped slit field constructed by combining an optical lens and a reflecting mirror so as to accurately control the velocity ratio between the mask (or reticle) and the wafer to be 4:1 during scanning exposure. The above-mentioned patent document (3) discloses an apparatus in which an excimer laser is used as an illumination light beam, and an effective projection area is subjected to restriction to a regular hexagon which is inscribed in a circular image field of an ordinary reduction projection lens system to perform exposure based on the SandS system. The above-mentioned patent document (4) discloses an apparatus in which an effective projection area is subjected to restriction to a linear slit-shaped (rectangular) area which is formed along a diameter of a circular image field of an ordinary reduction projection lens system to perform exposure based on the SandS system.
Besides, in order to obtain a higher resolving power, Japanese Laid-Open Patent Publication No. 6-300973 (5) discloses a reduction projection optical system comprising a plurality of optical lenses, a beam splitter, and a concave mirror, the system being applied to an ArF excimer laser having a wavelength of not more than 200 nm as an illumination light beam for exposure. A similar projection optical system is also disclosed in Japanese Laid-Open Patent Publication No. 5-88087 applied by the same applicant as that of the present application.
In the respective conventional techniques described above, the scanning type exposure apparatus based on the use of the reduction projection optical system generally adopts the system in which the reticle stage for holding the reticle and the wafer stage for holding the wafer are moved in a scanning manner at a velocity ratio which coincides with a reciprocal number of a reduction magnification of the projection optical system. Therefore, it has been necessary that a driving source (for example, a linear motor) for the reticle stage and a driving source (for example, a linear motor) for the wafer stage should be individually provided on an apparatus body (for example, a column for fixing the projection optical system thereon) to precisely control the both driving sources in a synchronized manner so that the reticle and the wafer are relatively moved while maintaining a constant velocity ratio. Namely, such a system requires the linear motor for linearly moving the reticle stage with respect to the projection optical system during scanning exposure, the linear motor for linearly moving the wafer stage with respect to the projection optical system, and a servo control circuit for individually and precisely controlling the respective linear motors on the basis of measured values obtained by laser interferometers for individually measuring the position of movement of the respective stages with respect to the projection optical system.
In the case of the scanning type exposure apparatus based on the use of the reduction projection optical system as described above, a method is generally adopted in which a stage having superior characteristics runs after a stage having inferior characteristics, because the reticle stage (mask stage) and the wafer stage (substrate stage) have different dynamic characteristics respectively. However, such a scanning type exposure apparatus has the following inconvenience. Namely, the stage having inferior characteristics is slow in setting adjustment, because it is affected, for example, by fluctuation of the body so serve as a backbone of the apparatus. In order to improve the synchronization performance, it is necessary to use a stage having extremely excellent dynamic characteristics, as the stage having superior characteristics. Moreover, it is necessary and indispensable to use a special unit such as a so-call active vibration-removing apparatus (vibration-preventive apparatus) for reducing fluctuation of the body. Therefore, the system of the apparatus is complicated to that extent, and the cost becomes expensive.
Further, the scanning exposure apparatus based on the use of the reduction projection system, in which the optical axis ranging from the reticle to the wafer is linear, has the following inconvenience, as in the apparatuses described in the foregoing patent documents (3) and (4). Namely, the reticle stage and the wafer stage are generally arranged so that both of them are moved in the horizontal direction. Further, the reticle stage and the wafer stage are disposed so that they are separated from each other in the vertical direction by a distance of about 80 to 150 cm. Accordingly, the reticle stage is arranged at an upper position of the body of the exposure apparatus. Therefore, the entire apparatus may be inclined due to scanning movement of the reticle stage during scanning exposure in some cases. In other cases, excessive stress may be applied to the respective structural components (for example, the column, and the base plate) for constituting the apparatus body.
The conventional apparatus has also involved the following inconveniences. Namely, disorder occurs in synchronous control for the linear motor for the reticle stage and the linear motor for the wafer stage. Further, the transfer magnification becomes non-uniform in relation to the scanning direction concerning the pattern image transferred to the shot area on the wafer if the interferometer suffers measurement error (count mistake).
The present invention has been made considering the inconveniences involved in the conventional technique as described above, an object of which is to provide an exposure apparatus and a scanning exposure apparatus each of which has a simple structure and makes it possible to reduce the stress generated in the structural components for constructing the apparatus, suppress inclination and fluctuation of the entire apparatus, and improve the synchronization performance of the mask stage and the substrate stage.
Another object of the present invention is to provide a scanning exposure method which makes it possible to reduce the stress generated in the exposure apparatus, suppress inclination and fluctuation of the entire apparatus, and improve the synchronization performance of the mask stage and the substrate stage.
According to a first aspect of the present invention, there is provided an exposure apparatus for transferring a pattern formed on a mask onto a photosensitive substrate through a projection optical system while synchronously moving the mask and the photosensitive substrate, the exposure apparatus comprising:
a substrate stage supported in a floating manner over a base member;
a mask stage supported in a floating manner over the base member and having a mass corresponding to an amount obtained by multiplying a mass of the substrate stage by a reduction magnification of the projection optical system; and
a first linear motor provided between the both stages, for driving the substrate stage and the mask stage so that the mask and the substrate are moved in mutually opposite directions.
According to the exposure apparatus, the mask stage and the substrate stage are supported in the floating manner on the base member. Therefore, the both stages are driven by the first linear motor in the mutually opposite directions while making no contact along the movement direction. During this process, no force is exerted at all on the base member and other components by the movement of the both stages. Thus, the momentum is conserved. In the present invention, the mass of the mask stage corresponds to the amount obtained by magnifying the mass of the substrate stage by the reduction magnification. Therefore, according to the law of conservation of momentum, the velocity ratio between the mask stage and the substrate stage is a reciprocal number of the reduction magnification of the projection optical system, and thus the both stages are subjected to accurate synchronous control. The position of the center of gravity of the entire system scarcely changes, and hence the main body including the base member is neither fluctuated nor inclined due to the movement of the both stages (synchronous scanning for the mask and the substrate).
The exposure apparatus of the present invention may be constructed such that the projection optical system is an optical system which projects an inverted image of the pattern formed on the mask onto the photosensitive substrate. When the exposure apparatus is constructed as described above, the photosensitive substrate is accurately exposed by projection with the image of the pattern when the mask stage and the substrate stage are synchronously moved in the mutually opposite directions by the aid of the first linear motor.
The exposure apparatus of the present invention may be constructed such that the photosensitive substrate is held horizontally on the substrate stage, the mask is held vertically on the mask stage, and the projection optical system comprises a plurality of transmitting optical elements, a beam splitter, and a reflecting optical element, wherein the optical system projects the pattern of the mask arranged on an object plane onto the photosensitive substrate arranged on an image formation plane at a predetermined reduction magnification. When the exposure apparatus is constructed as described above, for example, it is possible to arrange a half TTL alignment detection system on a side opposite to the mask stage in relation to the beam splitter. By doing so, the alignment mark formed on the mask can be detected separately from or simultaneously with the alignment mark formed on the photosensitive substrate by using the half TTL alignment detection system by the aid of the beam splitter. Accordingly, the detection of the reticle alignment mark and the detection of the wafer alignment mark can be conveniently performed by using the single detecting system.
The exposure apparatus of the present invention may be constructed such that a second linear motor for driving the mask stage is provided between the base member and the mask stage. According to this structure, the mask stage can be driven independently from the base member by driving the second linear motor in a state in which the first linear motor is turned OFF. Accordingly, it is possible to perform positional resetting and fine adjustment for the mask stage.
The exposure apparatus of the present invention may be constructed such that a third linear motor for driving the substrate stage is provided between the base member and the substrate stage. When the exposure apparatus is constructed as described above, the substrate stage can be driven independently from the base member by driving the third linear motor in a state in which the first linear motor (and the second linear motor) is/are turned OFF. Accordingly, it is possible to perform positional resetting and fine adjustment for the substrate stage.
The exposure apparatus of the present invention may be constructed such that a regenerative braking circuit for finely adjusting a velocity ratio during synchronous movement of the both stages effected by the first linear motor is provided together with at least one of the second linear motor and the third linear motor. When the exposure apparatus is constructed as described above, at least one of the second linear motor and the third linear motor is allowed to perform the regenerative braking action by the aid of the regenerative braking circuit. Thus, the apparent mass of at least any one of the mask stage and the substrate stage which are moved in the mutually opposite directions by the first linear motor can be increased to finely adjust the velocity ratio between the both stages during the movement. The term xe2x80x9cregenerative brakingxe2x80x9d refers to occurrence of the braking action brought about by allowing the motor to function as a kind of generator. Accordingly, the load to be driven by the first linear motor can be increased. Namely, it is possible to increase the apparent mass of at least one of the mask stage and the substrate stage. According to the exposure apparatus in conformity with the foregoing construction, when the mass ratio between the both stages is not accurately set to be a desired value, the velocity ratio between the both stages can be adjusted to ensure desired synchronization performance. Besides, it is possible to ensure stable synchronization performance even when the momentum is not completely conserved by always making the velocity ratio between the both stages to coincide with the reciprocal number of the reduction magnification of the projection optical system by appropriately adjusting the regenerative braking amount during the movement (scanning).
The exposure apparatus of the present invention may be constructed such that the substrate stage comprises a first stage which is movable in a first direction in which the photosensitive substrate is synchronously moved, and a second stage which is movable in the first direction integrally with the first stage while holding the photosensitive substrate and which is movable in a second direction perpendicular to the first direction by being guided by the first stage. When the exposure apparatus is constructed as described above, the second stage for holding the substrate is moved in the first direction integrally with the first stage to perform scanning exposure, and then the second stage is moved in the second direction perpendicular to the first direction with respect to the first stage. By repeating this process, it is possible to easily realize exposure based on the so-called step-and-scan system.
According to a second aspect of the present invention, there is provided a scanning exposure apparatus including a projection optical system having an optical axis substantially perpendicular to a mask and a substrate respectively, for transferring a pattern formed on the mask onto the substrate through the projection optical system, the scanning exposure apparatus comprising:
a base;
a first stage for moving the mask over the base;
a second stage for moving the substrate over the base; and
a driving system connected to the first stage and the second stage, for synchronously moving the mask and the substrate at a velocity ratio corresponding to a magnification of the projection optical system, wherein:
the driving system drives the first stage and the second stage along predetermined directions oppositely to one another so that a reactive force generated by the synchronous movement is offset.
According to a third aspect of the present invention, there is provided a scanning exposure method for transferring a pattern formed on a mask onto a substrate through a projection optical system, the scanning exposure method comprising the steps of:
arranging the mask and the substrate in an identical plane perpendicular to an optical axis of the projection optical system;
projecting a partial inverted image of the pattern formed on the mask onto the substrate; and
synchronously moving the mask and the substrate oppositely to one another along predetermined directions on the plane so that a reactive force generated by the synchronous movement is substantially offset.